In the manufacture of concentrated phosphate fertilizer, the phosphate ore is initially subject to extraction with sulfuric acid by the "wet process". The resulting dilute phosphoric acid is concentrated by boiling and then used for the production of concentrated liquid and granular phosphate fertilizers. During the extraction of phosphoric acid from the phosphase ore by sulfuric acid, the reaction produces hydrogen fluoride. In the presence of silica, the hydrogen fluoride is converted to fluosilicic acid.
A large quantity of the fluorine remains in the dilute phosphoric acid as hydrogen fluoride and fluosilicic acid. When the dilute phosphoric acid is concentrated by boiling, much of the fluorine is evolved with the steam as hydrogen fluoride and silicon tetrafluoride vapors in variable amounts depending upon the variety of the crude phosphate ore. The toxicity of both compounds in itself demands careful purification of the waste gases. Furthermore, the waste gases form a valuable raw material source for obtaining fluorine values in the form of inorganic fluorine compounds. From some phosphate plants, as much as 10,000 to 30,000 tons per year of fluorine compounds may be liberated in gaseous plant effluents, which should be captured to prevent air and water contamination. In recent years, both air and surface water pollution laws and regulations have been more stringent and have been more vigorously enforced. The present invention contributes to the prevention of contamination of air and surface water by various inorganic fluorine compounds being produced during the extraction of phosphate rock.
A principle object of the present invention is to provide an apparatus for recovering inorganic fluorine compounds from phosphate plants. An additional objective of the present invention is to control by recovery, the gaseous fluorine containing vapors produced during the concentration of phosphoric acid. A further objective of the invention is to provide an apparatus and process for recovery of fluorine in a valuable useable form.
Other objects and advantages of the invention will appear from the following description of preferred embodiments of the invention, with reference to the accompanying drawings showing the improved condenser and overall schematic flow diagram of the invention.